What copper-based pigment makes horseshoe crab blood appear blue?

Hemocyanin makes horseshoe crab blood appear blue.

Hemocyanin is a respiratory protein that uses copper atoms to bind oxygen. When oxygenated, it has a blue color. Horseshoe crabs circulate hemolymph rather than vertebrate-style blood, and their hemocyanin transports oxygen through the body.

Horseshoe crab blood is also medically important because cells in its hemolymph can detect bacterial endotoxins. Compounds derived from these cells have historically been used to test injectable drugs, vaccines and medical devices for contamination. Modern alternatives and recombinant methods are increasingly used to reduce harvesting pressure.

The blue color is not unique to horseshoe crabs: hemocyanin also occurs in many mollusks and crustaceans. Human blood is red because its oxygen-carrying protein, hemoglobin, contains iron. The crab’s blue hemolymph therefore reflects chemistry, not unusually cold blood or an absence of oxygen.